Pages

AUTO ALARM

In operation, the alarm circuit allows a 0 - 47 second time
delay, as determined by the R1 /C1 combination, after the switch is armed to
allow the vehicle's motion sensor to settle down. This allows you time to get a
bag of groceries out of the trunk and not have the hassle of juggling the
groceries and the key switch at once.

During the time delay, half of LED1, which is actually a
single, bi-colored, three-legged common cathode device, lights green. At the
same time, pins 8 and 4 of U2 (a 555 oscillator/timer) are held low by Ul (a
3905 oscillator/timer), causing the alarm to remain silent. Once the delay is
over, LED1 turns red, indicating that the circuit is armed.

At that point, a
ground at pin 2 of U2 forces pin 3 of U2 high, closing the contacts of Kl and
sounding the siren for a time duration determined by R4 and C2. Once the time
has elapsed, pin 3 is pulled low, Kl opens, and the circuit is again ready to
go. The circuit can be manually reset by the simple expedient of opening and
closing the key switch. Potentiometer R3 controls the LED's illumination
intensity. Diode D1 ensures that the green segment of LED1 is fully extinguished
when Ql is turned on-which turns the LED to red. Resistors R4 and R5 must be
connected to the + V bus. not to pin 7 of Ul. otherwise U2 will mysteriously
trigger itself each time the initial delay ends.